The invention relates to methods for reducing the phenotypic expression of a nucleic acid of interest in plant cells by providing aberrant RNA molecules, preferably unpolyadenylated RNA molecules comprising at least one target specific nucleotide sequence homologous to the nucleic acid of interest, preferably a sense strand, into the nucleus of plant cells. The target-specific unpolyadenylated RNA molecules may be provided by introduction of chimeric DNAs which when transcribed under control of conventional promoter and 3xe2x80x2 end formation and polyadenylation regions yield RNA molecules wherein at least the polyadenylation signal may be removed by the autocatalytic activity of a self-splicing ribozyme comprised within the transcribed RNA molecules. Also provided are plant cells comprising such RNA molecules or chimeric DNA encoding such RNA molecules, as well as plants. Similar methods and means for reducing the phenotypic expression of a nucleic acid by co-suppression in eukaryotic cells are provided.
Post-transcriptional gene silencing (PTGS) or co-suppression, is a common phenomenon associated with transgenes in transgenic plants. PTGS results in sequence-specific removal of the silenced transgene RNA as well as homologous endogenous gene RNA or viral RNA. It is characterized by low steady-state mRNA levels with normal (usually high) rates of nuclear transcription of transgenes being maintained. There are a number of common features or characteristics for PTGS. PTGS is
sequence-specific;
systemically transmissible;
often associated with the presence of multiple copies of transgenes or with the use of strong promoters;
frequently correlated with the presence of repetitive DNA structures, including inverted repeat T-DNA insertion patterns;
often accompanied by de novo DNA methylation in the transcribed region, and
may be meiotically reset.
A number of hypothetical models have been proposed to explain PTGS (see e.g. Wassenegger and Pxc3xa9lissier, 1998). Typically, these models suggest the involvement of a host encoded enzyme (RNA-directed RNA polymerase (RdRP)) which is proposed to use aberrant RNA as templates to synthesize small copy RNA molecules (cRNA). These cRNAs would then hybridize with the target mRNA to form duplex structures, thereby rendering the mRNA susceptible to degradation by endoribonucleases. So far, there has been no direct evidence that RdRP is involved in PTGS in plants.
An important question arising from the existing models is what type of RNA is the aberrant RNA that would be used as a template by RdRP, and in which cellular compartment RdRP would function.
Several reports have described the accumulation of unproductive or unpolyadenylated transgene RNA in plants which are post-transcriptionally silenced (Lee et al. 1997; van Eldik et al. 1998; Covey et al., 1997; van Houdt et al., 1997; Metzlaff et al.; 1997).
The following documents relate to methods and means for regulating or inhibiting gene expression in a cell.
U.S. Pat. No. 5,190,131 and EP 0 467 349 A1 describe methods and means to regulate or inhibit gene expression in a cell by incorporating into or associating with the genetic material of the cell a non-native nucleic acid sequence which is transcribed to produce an mRNA which is complementary to and capable of binding to the mRNA produced by the genetic material of that cell.
EP 0 223 399 A1 describes methods to effect useful somatic changes in plants by causing the transcription in the plant cells of negative RNA strands which are substantially complementary to a target RNA strand. The target RNA strand can be a mRNA transcript created in gene expression, a viral RNA, or other RNA present in the plant cells. The negative RNA strand is complementary to at least a portion of the target RNA strand to inhibit its activity in vivo.
EP 0 240 208 describes a method to regulate expression of genes encoded for in plant cell genomes, achieved by integration of a gene under the transcriptional control of a promoter which is functional in the host and in which the transcribed strand of DNA is complementary to the strand of DNA that is transcribed from the endogenous gene(s) one wishes to regulate.
EP 0 647 715 A1 and U.S. Pat. Nos. 5,034,323, 5,231,020 and 5,283,184 describe methods and means for producing plants exhibiting desired phenotypic traits, by selecting transgenotes that comprise a DNA segment operably linked to a promoter, wherein transcription products of the segment are substantially homologous to corresponding transcripts of endogenous genes, particularly endogenous flavonoid biosynthetic pathway genes.
Waterhouse et al. 1998 describe that virus resistance and gene silencing in plants can be induced by simultaneous expression of sense and anti-sense RNA. The sense and antisense RNA may be located in one transcript that has self-complementarity.
Hamilton et al. 1998 describes that a transgene with repeated DNA, i.e. inverted copies of its 5xe2x80x2 untranslated region, causes high frequency, post-transcriptional suppression of ACC-oxidase expression in tomato.
WO 98/53083 describes constructs and methods for enhancing the inhibition of a target gene within an organism which involve inserting into the gene silencing vector an inverted repeat sequence of all or part of a polynucleotide region within the vector.
WO 95/34688 describes methods for cytoplasmic inhibition of gene expression and provides genetic constructs for the expression of inhibitory RNA in the cytoplasm of eukaryotic cells. The inhibitory RNA may be an anti-sense or a co-suppressor RNA. The genetic constructs are capable of replicating in the cytoplasm of a eukaryotic cell and comprise a promoter region, which may be a plant virus subgenomic promoter in functional combination with the RNA encoding region.
WO095/15394 and U.S. Pat. No. 5,908,779 describe a method and construct for regulating gene expression through inhibition by nuclear antisense RNA in (mouse) cells. The construct comprises a promoter, antisense sequences, and a cis-or trans-ribozyme which generates 3xe2x80x2-ends independently of the polyadenylation machinery and thereby inhibits the transport of the RNA molecule to the cytoplasm.
The present invention provides a method for reducing the phenotypic expression of a nucleic acid of interest, which is normally capable of being expressed in a plant cell, the method comprising the step of providing to the nucleus of that plant cell aberrant RNA comprising a target-specific nucleotide sequence, preferably unpolyadenylated RNA comprising a target specific nucleotide sequence, particularly by producing aberrant RNA such as unpolyadenylated RNA by transcription of a chimeric DNA comprised within the plant cell, the chimeric DNA comprising a plant-expressible promoter operably linked to a target specific DNA region encoding that RNA and optionally further comprising a DNA region involved in 3xe2x80x2 end formation and polyadenylation, preceded by a self-splicing ribozyme encoding DNA region.
The invention also provides a method for reducing the phenotypic expression of a nucleic acid of interest, which is normally capable of being expressed in a plant cell, the method comprising the step of introducing into the nuclear genome of the plant cell a chimeric DNA to generate a transgenic plant cell, the chimeric DNA comprising the following operably linked parts:
a plant-expressible promoter region, preferably a constitutive promoter or an inducible promoter or a tissue-specific promoter;
a target-specific DNA region encoding a target-specific nucleotide sequence, preferably a target-specific DNA region comprising a nucleotide sequence of at least 10 consecutive nucleotides having at least about 70% sequence identity to about 100% sequence identity to the nucleic acid of interest or comprising a nucleotide sequence of at least 10 consecutive nucleotides having at least about 70% sequence identity to about 100% sequence identity to the complement of said nucleic acid of interest;
a DNA region encoding a self-splicing ribozyme, preferably a self-splicing ribozyme comprising a CDNA copy of a self-splicing ribozyme from avocado sunblotch viroid, peach latent mosaic viroid, Chrysanthemum chlorotic mottle viroid, carnation stunt associated viroid, Newt satellite 2 transcript, Neurospora VS RNA, barley yellow dwarf virus satellite RNA,arabis mosaic virus satellite RNA, chicory yellow mottle virus satellite RNA S1, lucerne transient streak virus satellite RNA, tobacco ringspot virus satellite RNA, subterranean clover mottle virus satellite RNA, solanum nodiflorum mottle virus satellite RNA, velvet tobacco mottle virus satellite RNA, Cherry small circular viroid-like RNA or hepatitis delta virus RNA, particularly a DNA region comprising the nucleotide sequence of SEQ ID No 1 or SEQ ID No 2 or a ribozyme-effective part thereof; and
a) a DNA region involved in 3xe2x80x2 end formation and polyadenylation;
wherein said chimeric DNA when transcribed produces a first RNA molecule comprising a target specific nucleotide sequence and a self-splicing ribozyme, which when cleaved by autocatalysis produces a second RNA molecule comprising a target specific nucleotide sequence wherein the 3xe2x80x2 end of the first RNA molecule comprising the polyadenylation site has been removed. Optionally, a transgenic plant may be regenerated from the transgenic plant cell. Preferably, the DNA region encoding a self-splicing ribozyme is located immediately upstream of the DNA region involved in 3xe2x80x2 end formation and polyadenylation.
It is another objective of the invention to provide a chimeric DNA molecule for reducing the phenotypic expression of a nucleic acid of interest, which is normally capable of being expressed in a plant cell, comprising
a plant-expressible promoter region, preferably a constitutive promoter or an inducible promoter or a tissue-specific promoter;
a target-specific DNA region encoding a target-specific nucleotide sequence, preferably a target-specific DNA region comprising a nucleotide sequence of at least 10 consecutive nucleotides having at least about 70% sequence identity to about 100% sequence identity to the nucleic acid of interest or comprising a nucleotide sequence of at least 10 consecutive nucleotides having at least about 70% sequence identity to about 100% sequence identity to the complement of said nucleic acid of interest;
a DNA region encoding a self-splicing ribozyme, preferably a self-splicing ribozyme comprising a cDNA copy of a self-splicing ribozyme from avocado sunblotch viroid, peach latent mosaic viroid, Chrysanthemum chlorotic mottle viroid, carnation stunt associated viroid, Newt satellite 2 transcript, Neurospora VS RNA, barley yellow dwarf virus satellite RNA,arabis mosaic virus satellite RNA, chicory yellow mottle virus satellite RNA S1, lucerne transient streak virus satellite RNA, tobacco ringspot virus satellite RNA, subterranean clover mottle virus satellite RNA, solanum nodiflorum mottle virus satellite RNA, velvet tobacco mottle virus satellite RNA, Cherry small circular viroid-like RNA or hepatitis delta virus RNA, particularly a DNA region comprising the nucleotide sequence of SEQ ID No 1 or SEQ ID No 2 or a ribozyme-effective part thereof; and
a DNA region involved in 3xe2x80x2 end formation and polyadenylation;
wherein said chimeric DNA when transcribed produces a first RNA molecule comprising a target specific nucleotide sequence and a self-splicing ribozyme, which when cleaved by autocatalysis produces a second RNA molecule comprising a target specific nucleotide sequence wherein the 3xe2x80x2 end of the first RNA molecule comprising the polyadenylation site has been removed. Preferably, the DNA region encoding a self-splicing ribozyme is located immediately upstream of the DNA region involved in 3xe2x80x2 end formation and polyadenylation.
It is yet another objective of the invention to provide plant cells and plants comprising a nucleic acid of interest which is normally capable of being phenotypically expressed, further comprising a chimeric DNA, preferably stably integrated into the nuclear genome, comprising
a plant-expressible promoter region, preferably a constitutive promoter or an inducible promoter or a tissue-specific promoter;
a target-specific DNA region encoding a target-specific nucleotide sequence, preferably a target-specific DNA region comprising a nucleotide sequence of at least 10 consecutive nucleotides having at least about 70% sequence identity to about 100% sequence identity to the nucleic acid of interest or comprising a nucleotide sequence of at least 10 consecutive nucleotides having at least about 70% sequence identity to about 100% sequence identity to the complement of said nucleic acid of interest;
a DNA region encoding a self-splicing ribozyme, preferably a self-splicing ribozyme comprising a cDNA copy of a self-splicing ribozyme from avocado sunblotch viroid, peach latent mosaic viroid, Chrysanthemum chlorotic mottle viroid, carnation stunt associated viroid, Newt satellite 2 transcript, Neurospora VS RNA, barley yellow dwarf virus satellite RNA,arabis mosaic virus satellite RNA, chicory yellow mottle virus satellite RNA S1, lucerne transient streak virus satellite RNA, tobacco ringspot virus satellite RNA, subterranean clover mottle virus satellite RNA, solanum nodiflorum mottle virus satellite RNA, velvet tobacco mottle virus satellite RNA, Cherry small circular viroid-like RNA or hepatitis delta virus RNA, particularly a DNA region comprising the nucleotide sequence of SEQ ID No 1 or SEQ ID No 2 or a ribozyme-effective part thereof; and
a DNA region involved in 3xe2x80x2 end formation and polyadenylation;
wherein said chimeric DNA when transcribed produces a first RNA molecule comprising a target specific nucleotide sequence and a self-splicing ribozyme, which when cleaved by autocatalysis produces a second RNA molecule comprising a target specific nucleotide sequence wherein the 3xe2x80x2 end of the first RNA molecule comprising the polyadenylation site has been removed.
The invention also provides a method for identifying a phenotype associated with the expression of a nucleic acid of interest in a plant cell, the method comprising:
1) selecting within the nucleic acid of interest a target sequence of at least 10 consecutive nucleotides;
2) introducing a chimeric DNA into the nucleus of a suitable plant host cell comprising the nucleic acid of interest, the chimeric DNA comprising the following operably linked DNA fragments:
a) a plant-expressible promoter region;
b) a target-specific DNA region comprising a nucleotide sequence of at least about 70% to about 100% sequence identity to said target sequence or to the complement of said target sequence; followed by
c) a DNA region encoding a self-splicing ribozyme located immediately upstream of
d) a DNA region involved in 3xe2x80x2 end formation and polyadenylation;
3) observing the phenotype by a suitable method.
Yet another objective of the invention is to provide a method for reducing the phenotypic expression of a nucleic acid of interest, which is normally capable of being expressed in a eukaryotic cell, the method comprising the step of providing to the nucleus of said eukaryotic cell aberrant RNA, preferably unpolyadenylated RNA, comprising a target specific nucleotide sequence of at least 10 consecutive nucleotides with at least about 70% sequence identity to about 100% sequence identity to the nucleotide sequence of the nucleic acid of interest, paritucularly by producing aberrant RNA such as unpolyadenylated RNA by transcription of a chimeric DNA comprised within the eukaryotic cell, the chimeric DNA comprising a plant-expressible promoter operably linked to a target specific DNA region encoding that RNA and optionally further comprising a DNA region involved in 3xe2x80x2 end formation and polyadenylation, preceded by a self-splicing ribozyme encoding DNA region.
Still another objective of the invention is to provide a method for reducing the phenotypic expression of a nucleic acid of interest, which is normally capable of being expressed in a eukaryotic cell, comprising the step of introducing into the nuclear genome of the eukaryotic cell a chimeric DNA to generate a transgenic plant cell, comprising the following operably linked parts:
a) a promoter region functional in the eukaryotic cell;
b) a target-specific DNA region comprising nucleotide sequence of at least 10 consecutive nucleotides with at least about 70% sequence identity to about 100% sequence identity to the nucleotide sequence of the nucleic acid of interest;
c) a DNA region encoding a self-splicing ribozyme; and
d) a DNA region involved in 3xe2x80x2 end formation and polyadenylation
wherein the chimeric DNA when transcribed produces a first RNA molecule comprising a target specific nucleotide sequence and a self-splicing ribozyme, which when cleaved by autocatalysis produces a second RNA molecule comprising a target specific nucleotide sequence wherein the 3xe2x80x2 end of the first RNA molecule comprising the polyadenylation site has been removed.
The invention also provides a eukaryotic cell comprising a nucleic acid of interest, normally capable of being phenotypically expressed, further comprising a chimeric DNA comprising the following operably linked parts:
a promoter region functional in the eukaryotic cell;
a target-specific DNA region comprising nucleotide sequence of at least 10 consecutive nucleotides with at least about 70% sequence identity to about 100% sequence identity to the nucleotide sequence of the nucleic acid of interest;
a DNA region encoding a self-splicing ribozyme; and
a DNA region involved in 3xe2x80x2 end formation and polyadenylation
wherein said chimeric DNA when transcribed in the eukaryotic cell produces a first RNA molecule comprising a target specific nucleotide sequence and a self-splicing ribozyme, which when cleaved by autocatalysis produces a second RNA molecule comprising a target specific nucleotide sequence wherein the 3xe2x80x2 end of the first RNA molecule comprising the polyadenylation site has been removed, as well as non-human eukaryotic organisms comprising or consisting essentially of such eukaryotic cells.